PHILADELPHIA (AP) — The NFL and former players must try to negotiate a dispute over whether complaints about concussion-related injuries belong in court or in arbitration, a federal judge said. U.S. District Judge Anita B. Brody of Philadelphia had planned to rule July 22 in a legal fight that involves about 4,200 former players and could be worth billions of dollars.

Instead, she ordered the two sides to begin mediation with retired federal Judge Layn Phillips. The retirees want the right to sue the league, while the NFL insists the claims must go to arbitration under terms of the collective bargaining agreement. Brody asked for a progress report by Sept. 3, while placing a gag order on the lawyers. Both sides agreed to comply. Many former players say they suffer from dementia, Alzheimer’s disease and other neurological conditions which they believe stem from on-field concussions. The league insists that safety has always been a top priority. Both sides have something to fear from a court ruling. Brody could throw out the lawsuits and steer them to arbitration or accept some or all of them and open up internal NFL files to plaintiffs’ lawyers looking for a smoking gun. Still, sports law professor Marc Edelman, who teaches at Fordham University and Baruch College, called it “highly unlikely” that either side would budge during mediation. “The sentimental impact of this type of case is one that would make it strongly advantageous for the plaintiffs to get to a jury,” Edelman said. “The position the NFL has taken is they are not liable for anything that’s happened to the players.” In legal arguments before Brody in April, NFL lawyer Paul Clement said teams bear the chief responsibility for health and safety under the contract, along with the players’ union and the players themselves. Players’ lawyer David Frederick accused the league of concealing studies linking concussions to neurological problems for decades. “The plaintiffs got in a lot of their case, that the NFL glorified violence all these years. I would think in mediation that Judge Phillips would let it go further, if that’s what they want to talk about,” said Andrew Brandt, who directs a sports law center at Villanova University School of Law. In recent years, a string of former NFL players and other concussed athletes have been diagnosed after their deaths with chronic traumatic encephalopathy, or CTE, including popular Pro Bowler Junior Seau and lead plaintiff Ray Easterling. Both committed suicide last year. About one-third of the league’s 12,000 former players have joined the litigation since 2011. Some legal experts feel the NFL may be most vulnerable on claims from a few hundred “gap” players, who played during years when there was no contract in place. “We respect and will comply with the court’s order regarding mediation and will be available to meet with Judge Phillips at his direction,” NFL spokesman Greg Aiello said in a statement. The Plaintiffs’ Executive Committee for the former NFL players also pledged to “follow the court’s directive.”